1. Field of the Invention
This invention relates to pumps and, more particulary, to submersible pumps.
2. Description of Related Art
Submersible pumps are designed to be immersed in the liquid that they will be pumping. One application of such pumps, for example, is in conjunction with aquarium undergravel filters. In an aquarium, it is necessary to treat the water to remove particulates and toxic chemicals and gases and to replenish the oxygen supply throughout the aquarium tank water. In undergravel filtration, a porous support plate is positioned slightly above the bottom of the aquarium tank. A layer of filtration material, such as gravel, is deposited on the support plate, forming a filtration layer. The aquarium water is circulated downwardly through the filtration material and porous support plate so that waste products in the water will be collected by the filtration material. After water is forced downwardly through the filtration layer, it is carried upward in a conduit passing through the filtration layer. At the top end of the conduit, the water is discharged back into the aquarium tank. A submersible water pump is placed in the aquarium tank to provide this flow of water.
Typically, such submersible pumps comprise a rotating impeller, the impeller being attached to the rotor of an electric motor. The pump typically includes a motor housing, which is immersed in the aquarium tank. The motor housing includes a motor cavity, which accepts the motor coils comprising a stator assembly. An impeller housing receives the impeller and is attached to the motor housing. Attached to the impeller housing are an intake conduit for drawing in aquarium water through the undergravel filter and also a discharge conduit for directing the flow of water outwardly from the pump.
Because the motor housing is submerged in the aquarium tank, there is a danger that aquarium water may penetrate through the motor housing into the motor cavity and reach the motor coils, thereby destroying the motor. Therefore, it is common practice to fill the motor cavity space between the motor housing and the coils with a non-porous sealant material, or pottant, such as epoxy. In assembly, the coils are placed within the motor cavity, which is then filled with the epoxy sealant material. The epoxy pottant eventually cures into a hardened state. In this manner, a watertight seal is created around the coils. The curing time varies with the pottant material and the curing conditions.
Although the sealing method described above is generally effective, it does not provide complete assurance of obtaining a watertight seal. While the epoxy is curing, bubbles may form in the epoxy due to air around the coils being trapped within the motor cavity when the epoxy was poured. As the epoxy cures, the bubbles naturally float upward toward the surface of the epoxy. The bubbles may form a path or series of voids from the coils to the outside surface of the epoxy, allowing moisture to work its way from bubble to bubble toward the motor coils. Because of this problem, after the epoxy is cured, it must be inspected for bubble formation, resulting in some rejections for defects. It would be desirable to provide a submersible pump that is free of the safety and production problems due to bubble formation in the sealant material.